1. Field of the Invention
This invention relates to a process and apparatus for preparing crystalline powders of alphaaluminum orthophosphate and alpha-gallium orthophosphate.
2. Description of the Prior Art
Alpha-aluminum orthophosphate (berlinite) and alpha-gallium orthophosphate (GaPO.sub.4) are among several alpha-quartz isomorphs that have for decades been synthesized for research purposes. An attempt to grow large single crystals of berlinite began after World War II, in an effort to find new piezoelectric crystals for frequency control applications. The project ended a few years later, because success was achieved in quartz crystal growth and because quartz was considered superior for the piezoelectric devices known then. Specifically, it was concluded that berlinite had a lower Q and lower coupling coefficient than quartz. Furthermore, tests on both X and Y cuts of berlinite plates, showing a negative frequency drift with increasing temperature, indicated that there was little chance of finding a zero temperature cut similar to the AT cut of quartz.
Interest in berlinite was renewed in 1976, when Barsch and Chang found that berlinite does have temperature-compensated cuts, and that the coupling coefficient for surface acoustic wave (SAW) devices can be four times greater than for quartz.
Several processes for preparing berlinite have been reported in the technical literature (A. F. Huttenlocker, Z. Krist. 90, 508 (1935); W. Jahn et al., Chem. Erde 16, 75 (1953); and E. D. Kolb et al., J. Crystal Growth 43, 313 (1978)). Three processes for preparing GaPO.sub.4 were described by Perloff (J. Amer. Cer. Soc. 39, 83 (1956)).
Recently, Drafall and Belt, of the Rome Air Development Center, described procedures for seeded hydrothermal growth of single crystals of berlinite for SAW applications. (RADC-TR-80-73, Final Technical Report, March, 1980). Among the three approaches to nutrient preparation described in their report, the best results were achieved by reacting Al(OH).sub.3 with excess H.sub.3 PO.sub.4 according to the reaction: EQU Al(OH).sub.3 +H.sub.3 PO.sub.4 .fwdarw.AlPO.sub.4 +3H.sub.2 O
The amount of acid was adjusted so that the final solution was 6 M in H.sub.3 PO.sub.4. The reactants were heated in a sealed silver-lined autoclave under a gradient of 20.degree.-30.degree. C., with temperature increasing from 120.degree.-250.degree. C. at a fixed rate of about 5.degree. C./day, Euhedral crystals in the 1-4 mm range was produced.